HitFilm's compositor can work in 2D and 3D. Depending on the nature of your project you may use one or both of these modes.

3D should not be thought of as 'better'. Although it does provide more options for compositing, it is also more complex than 2D layer compositing. For many shots 2D compositing will be a faster and easier method.

2D composite shots can only have 2D layers.

3D composite shots can include both 2D and 3D layers.

Changing to 3D

To change a composite shot into 3D you need to add a camera. This can be done using the New Layer menu at the top of the composite shot timeline.

A default camera will be added and the scene will automatically switch into 3D.